1. Technical Field
The present invention relates generally to electronic circuits, and more specifically, the invention relates to switching power supplies.
2. Background Information
A common application of switching power supplies is to charge batteries. The output power of a battery charger is usually controlled to provide a regulated voltage and a regulated current. The voltage is regulated between a maximum and a minimum voltage over a range of output current. The current is regulated between a maximum and a minimum current over a range of output voltage. As the battery charges, there is usually m abrupt transition from regulated output current to regulated output voltage that occurs automatically when the battery voltage reaches a threshold. That is, the locus of output voltage and output current plotted in Cartesian coordinates usually has a sharp corner at the point of transition that corresponds to the point of maximum output power. Typically, there is also a requirement to substantially reduce the output current when the voltage falls below a threshold to prevent damage from a short circuit or similar fault on the output.
The practice of designing a battery charger to have a sharp transition between regulated voltage and regulated current can result in a product that costs more than necessary to provide the desired function. It is often possible to reduce the cost of the batten charger and to meet all requirements by designing an unregulated transition between the regulated voltage and the regulated current. The output voltage and output current in the region of unregulated transition is bounded by the natural output characteristics of the switching regulator, and typically follows the curve of maximum output power for a given output voltage or current.
To achieve lower cost, the switching regulator is designed to operate with a control circuit that permits the regulator to make an unregulated transition between regulated output voltage and regulated output current such that the voltage and current are maintained within the specified boundaries. Proper design of the unregulated transition within the specified boundaries reduces the maximum power output, allowing the use of components that are less costly than the components to guarantee higher output power. The control circuit operates the switching regulator for regulated voltage, regulated current, unregulated transition, or self-protection according to the magnitude of a feedback signal.
Battery chargers typically use one circuit to sense the output voltage and a different circuit to sense the output current for the purposes of regulation. In many applications, it is possible to eliminate the circuitry that senses output current, and to use an unregulated transition between the regulated output voltage and self-protection threshold voltage to satisfy the requirements of the design. Elimination of circuitry to sense output current reduces cost and raises efficiency.